package 机试题16年;

import java.text.DecimalFormat;
import java.util.Scanner;

/**
 * Created by hest0 on 2017/4/18.
 */
public class 蘑菇阵1 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            int n = sc.nextInt();
            int m = sc.nextInt();
            int k = sc.nextInt();

            int[][] map = new int[n + 1][m + 1];

            for (int i = 0; i < k; i++) {
                int x = sc.nextInt();
                int y = sc.nextInt();
                map[x][y] = 1;
            }

            double[][] dp = new double[n + 1][m + 1];
            dp[1][1] = 1;

            for (int i = 1; i < n + 1; i++) {
                for (int j = 1; j < m + 1; j++) {
                    if (i == 1 && j == 1) {
                        continue;
                    }
//                    if (map[i][j] == 1)
//                        dp[i][j] = 0;
//                    else
//                        dp[i][j] = (j == m ? 1 : 0.5) * dp[i - 1][j] + (i == n ? 1 : 0.5) * dp[i][j - 1];

                    dp[i][j] = (map[i][j] == 1) ? 0 : (j == m ? 1 : 0.5) * dp[i - 1][j] + (i == n ? 1 : 0.5) * dp[i][j - 1];


                }
            }

            System.out.println(new DecimalFormat("#0.00").format(dp[n][m]));

        }
        sc.close();
    }


}
